Directly heated dispenser cathode structure and the method of fabricating same



May 26, 1970 R. T. REAVES 3,514,661

DIRECTLY HEATED DISPENSER CATHODE STRUCTURE AND THE METHOD OF FABRICATING SAME Filed July 5, 1968 FIG 2A F|G.. 2B

l-6I00 3 INPUT 2 4O VOLTAGE 5o 1 l l l 1 l 900 I000 n00 I200 Temp 5 INVENTOR. F "3-8 BY RONALD T. REAVES I ATTORNEYS United States Patent fornia t Filed July 5, 1968, Ser. No. 742,814 Int. Cl. H01j 1/14, 19/06 U-S. Cl. 313-346 6 Claims ABSTRACT OF THE DISCLOSURE A self-supporting, directly heated, dispenser cathode is described and method of making same wherein grooves are cut in a body, filler material added to the grooves, the body counter bored, and the filler material removed.

The present invention is directed generally to a directly heated electron emitter and more particularly to a directly heated dispenser type cathode and method of making same.

Dispenser type cathodes have been known wherein a porous refractory metal body, usually tungsten, is provided with a supply of material within the pores to produce a continuous supply of material to a surface of the body connected with the pores for emission of electrons when the body is heated by means of an auxiliary heater coil. Typical dispenser type cathodes and their manufacture are described in the article Electron Tube Cathode by Kenneth Dudley, Proceedings of the Fifth National Conference on Advances on Electron Tube Techniques, pages 154-158, September 1960, and Koppius Pat. No. 3,076,916. Numerous problems are attendant such indirectly heated cathodes including heater short circuits, insulation breakdown, chipped heater cuttings, lead wire breakage, poor heat transfer, changing conduction char acteristics, heater and cathode assembly problems, etc.

Broadly stated, the present invention is directed to a directly heated, self-supporting electron emitter comprising a porous refractory metal body having an opening therein or structure defining a circuitous electrical conduction path through the body formed by impregnating the body with a material rendering the body machineable and machining the impregnated body to form the opening or structure, the material substantially filling the pores of the body to produce a continuous supply of material at a surface of the body connected with the pores for emission of electrons, and first and second electrically conductive members connected to said body on opposite sides of the opening spaced along the circuitous path whereby electrical current passing through the body between the conductive members heats the body for establishment of thermionic emission therefrom.

In amordance with one embodiment of the invention, the cathode body is in the form of a self-supporting helix formed by machining. This cathode is formed by machining a cylinder from the body impregnated for machineability and machining grooves in the cylindrical surface with the grooves in a helical pattern. These grooves are then filled with the same material making the body machineable and this body bored axially to remove all material radially inwardly of the grooves. When the material is removed from the pores and grooves, a helical body structure remains which can then be impregnated with a material capable of providing a continuous supply of electrons in the body surface.

Other-objects and advantages of this invention will become apparent when reading the following description and referring to the accompanying drawing in which simi- Patented May 26, 1970 "ice lar characters of reference represent corresponding parts in each of the several views.

In the drawings:

FIG. 1 is a plan view of a cathode constructed in accordance with the present invention;

FIGS. 2A-6A are plan views of structures during different stages of formation of the cathode of FIG. 1;

FIGS. 2B-6B are cross-sectional views of the structures shown in FIGS. 2A-6A FIG, 7 is a plan view of another embodiment of this invention; and V FIG. 8 is a graph showing the performance characteristics of a cathode in accordance with this invention.

Referring now to the drawing, with particular reference to FIG. 1, there is shown a self-supporting, directly heated cathode 10 constructed in accordance with the present invention. The cathode includes an electron emissive body 11 shaped in the form of a helix with a number of turns. The helix around the opening defined by the central bore 11' and spacing between turns 11" defines a circuitous electrical path for current to flow through and heat the body. The body is formed of refractory metal, typically tungsten, in the manner described in greater detail and a material such as barium calcium aluminate is provided in the body pores to produce upon application of heat a continuous supply of material capable of emitting electrons at the body surface. Electrically conductive members or lead wires 12 and 13, such as rhenium, are secured preferably before impregnation as by heliarc welds to the ends of the helix for direct application of electrical current to the cathode. If desired, the helical structure can be supported on support rods of an insulating material such as sapphire to aid in prevention of sagging of the turns.

This directly heated dispenser cathode avoids all problems associated with typical indirectly heated cathodes having separate heaters. It is particularly useful for high emission requirements such as in triodes, tetrodes, power rectifiers, hollow are devices and especially lasers.

The cathode 10 shown in FIG. 1 is fabricated by the series of steps illustrated by the structures shown in FIGS. 2-6. First, the tungsten body is formed by pressing tungsten powder under high pressure and then sintering the compressed body to produce a porous body, typically 70- dense, which is normally unmachineable to close tolerances. Next, the porous body is impregnated with a filler material such as a metal or plastic. In the case of a metal such as copper, the filler material can be melted into the pores while in the case of a plastic such as methyl methacrylate, the porous body can be impregnated with the liquid plastic which then sets up and hardens in the pores. This body is machined to a cylindrical shape as shown in FIGS. 2A and 2B.

Next, as shown in FIGS. 3A and 3B, threads or helical grooves 15 are cut in the cylindrical surface of the body 14 to define lands 15 of impregnated tungsten therebetween. These grooves 15 are then filled with material 16 (see FIGS. 4A and 4B) adhering to the lands, and preferably the same metal or plastic with which the body is impregnated is utilized. By then machining as by counter boring the body, the core of the body radially inward from the grooves is removed (see FIGS. 5A and 5B) leaving only the lands and adjacent filled grooves surrounding the bore 17. By then removing the filler material, such as by volatilization, the helical body remains (see FIGS. 6A and 6B) and can be impregnated with the material for electron emission. One such emitter material is barium calcium aluminate provided by melting together barium carbonate, calcium carbonate, and aluminum oxide, such as in mol ratios of 5:3:2, 4:111; and 3:1:1, and contacting the porous helical body with the melt. Cleaning the excess emitter material can easily be accomplished using aluminum oxide grit and sandblasting.

While the invention has been described with sufiicient detail to enable a person skilled in the art to manufacture and use the self-supporting directly heated cathode, the following illustrative example of one such cathode is given. A tungsten body is formed by pressing 99.9% pure tungsten powder, approximate average size of 5 microns, in a hydrostatic press at a pressure of approximately 18,000 p.s.i. This green compact is placed in a hydrogen furnace and heated at about 2300 C. for 20 minutes. These conditions can be modified slightly to produce the correct density which may be on the order of 80-83% in the preferred instance. Next, generally in the manner described in Pat. 3,076,916 referred to above methylmethacrylate is impregnated into the porous bar in liquid form and sets up and hardens in the pores. The impregnated metal can now be machined to the desired shape, and the plastic removed by volitalization. Lead wires are heliarced to the terminal ends of the structure and the body impregnated with emissive material barium calcium aluminate in the mol ratio of 4:1:1 in a hydrogen atmosphere at 1650 C. for 30 seconds. The body is cooled and excess emissive material can then be blasted off with aluminum oxide grit. A seven turn helical cathode with outside diameter .500 inch, inside diameter .420 inch, land width or turn thickness .040 inch, and turn spacing of 0.50 inch has performance characteristics as shown in FIG. 8.

Another embodiment of the present invention is shown in FIG. 7 which illustrates a cathode 20 with a ring shaped refractory metal body 21 impregnated with an emission material and provided With leads 22 and 23 on opposite sides of the central bore for opening 24. This cathode can be fabricated generally as shown in FIGS. 26 without the necessity for adding the filler material in the groove, but by adding the filler material in circular grooves a number of cathodes can be formed at one time.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is understood that certain changes and modifications may be practiced within the spirit of the invention as limited only by the scope of the appended claims.

What is claimed is:

1. The method of forming a self-supporting directly heatable cathode with large emission areas comprising the steps of forming a porous unmachineable refractory metal body; impregnating the pores of said body with a material that renders the body machineable; machining the impregnated body to remove first portions thereof; filling at least certain of said removed first body portions with a material joining the remaining body portions; machining the impregnated and filled body to remove second portions thereof; and removing the filling material from said first body portions.

2. The method of forming a self-supporting directly heatable cathode with large emission areas comprising the steps of forming a porous unmachineable refractory metal body; impregnating the pores of said body with a material that renders the body machineable; machining the impregnated body to remove first portions thereof; filling at least certain of said removed first body portions with a material joining the remaining body portions; machining the impregnated and filled body to remove second portions thereof of said body leaving remaining body portions which are joined together with said filling material; and removing said filling material.

3. The method of forming a self-supporting directly heatable cathode with large emissive areas comprising the steps of forming a porous tungsten body; melting a material into the pores of the body to render the body machineable; machining grooves in the impregnated tungsten body to define lands of impregnated tungsten therebetween; filling the cut grooves with said material; machining the interior of said tungsten body to remove all of the tungsten interiorly of the grooves with the only tungsten body material remaining contained in said lands; removing the material from the pores and grooves; and impregnating the pores of the remaining porous tungsten lands with an electron emitter.

4. A self-supporting directly heatable cathode with large emissive areas comprising a porous refractory metal body having an opening therein, said opening formed by impregnating said body with a material rendering the body machineable and machining said impregnated body to form said opening; a material substantially filling the pores of the body to produce a continuous supply of material at a surface of the body connected with said pores for emission of electrons; and first and second electrically conductive members connected to said body on opposite sides of said opening whereby electrical current passing through said body between said first and second members heats said body for establishment of thermionic emission therefrom.

5. A self-supporting directly heatable cathode with large emissive areas comprising a helical, porous refractory tungsten body having a plurality of turns around an axial opening, said helix formed by impregnating a cylindrical body with a material rendering the body machineable, machining helical turns in the cylindrical surface of said impregnated body, removing the body material radially inwardly from said turns, and removing said material; an emitter material substantially filling the pores of said body to produce a continuous supply of emitter material at a surface of said body connected with said pores for emission of electrons; a first electrically conductive member connected to one end of the helix forming said body; and a second electrically conductive member connected to the other end of the helix forming said body wherein electrical current passing through the body between said first and second members heats said body for establishment of thermionic emission therefrom.

6. A self-supporting directly heatable cathode with large emissive areas comprising a porous refractory metal body defining a circuitous path, said body formed by impregnating said body with a material rendering the body machineable and machining said impregnated body to form said path; a material substantially filling the pores of the body to produce a continuous supply of material at the surface of the body connected with said pores for emission of electrons; and first and second electrically conductive members connected to said body spaced apart along said path whereby electrical current passing through said body between said first and second members heats said body for establishment of thermionic emission therefrom.

References Cited UNITED STATES PATENTS 2,813,807 11/1957 Levi 313-346 X 2,848,644 8/ 8 Koppius 313346 3,076,919 2/1963 Koppius 313346 3,098,723 7/1963 Micks 3 l3341 3,269,804 8/1966 Affleck et al. 313346 JOHN W. HUCKERT, Primary Examiner A. I. JAMES, Assistant Examiner U.S. Cl. X.R. 

